Method for making ybco superconductor

ABSTRACT

A method of producing polycrystalline Y3Ba5Cu8Oy (Y-358) whereby powders of yttrium (III) oxide, a barium (II) salt, and copper (II) oxide are pelletized, calcined at 850 to 950° C. for 8 to 16 hours, ball milled under controlled conditions, pelletized again and sintered in an oxygen atmosphere at 900 to 1000° C. for up to 72 hours. The polycrystalline Y3Ba5Cu8Oy thus produced is in the form of elongated crystals having an average length of 2 to 10 μm and an average width of 1 to 2 μm, and embedded with spherical nanoparticles of yttrium deficient Y3Ba5Cu8Oy having an average diameter of 5 to 20 nm. The spherical nanoparticles are present as agglomerates having flower-like morphology with an average particles size of 30 to 60 nm. The ball milled polycrystalline Y3Ba5Cu8Oy prepared under controlled conditions shows significant enhancement of superconducting and flux pinning properties.

BACKGROUND OF THE DISCLOSURE Technical Field

The present disclosure relates to a method of producing apolycrystalline yttrium barium copper oxide (YBCO) material,specifically Y₃Ba₅Cu₈O_(y) (Y-358), using ball milling.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentdisclosure.

Yttrium barium copper oxide (YBCO) compounds are among the class ofhigh-Tc superconductors (HTS) that have been the subject of manyresearch studies. The most recognized YBCO superconductor isYBa₂Cu₃O_(y) (“Y-123”). This material exhibits unique characteristicsthat make it a promising candidate for electronic and magneticapplications, i.e., electric motors and power transmission, particleaccelerators, magnetic levitators devices, and transfer cablegenerators, etc. After the discovery of the superconductivity phenomenain Y-123, researchers have searched for novel phases of YBCO materialsthat exhibit higher superconducting transition temperature (T_(co)) andgreater superconducting properties than Y-123. Recently, a newyttrium-based superconductor with different characteristics andstructural properties, Y₃Ba₅Cu₈O_(18±δ) (“Y-358”), has been investigated[A. Aliabadi, Y. Akhavan Farshchi, M. Akhavan, Physica C 469 (2009)2012-2014.]. This material has five CuO₂ planes and three CuO chains(which exceed those in Y-123), and superconducting transitiontemperatures T_(co) in the range between 78 and 98 K. However, for usein practical applications, new methods are needed for producing Y-358with increased critical current density J_(c) when subject to a magneticfield by improving flux pinning.

The introduction of secondary phases inside the superconducting matrixis one way to improve flux pinning properties [E. Hannachi, Y. Slimani,F. Ben Azzouz, A. Ekicibil, Ceramics International (2018),https://doi.org/10.1016/j.ceramint.2018.07.118; R. A. Al-Mohsin, A. L.Al-Otaibi, M. A. Almessiere, H. Al-badairy, Y. Slimani, F. Ben Azzouz,Journal of Low Temperature Physics 192 (2018) 100-116; M. K. Ben Salem,E. Hannachi, Y. Slimani, A. Hamrita, M. Zouaoui, L. Bessais, M. BenSalem, F. Ben Azzouz, Ceramics International 40 (2014) 4953-4962.].While doping has achieved an improvement in the superconductingproperties of Y-358, the fabrication processes may introduce variousintermediate phases in equilibrium and thus influence the microstructureevolution of the material. Recently, high energy ball milling (HEBM) hasbeen used to alter the superconducting characteristics of MgB₂ compoundseither in bulk form or as tapes and wires [M. Shahabuddin, N. S.Alzayed, M. P. Jafar, M. Asif, Physica C 471 (2011) 1635; Z. Ma, Y. C.Liu, J. Huo, Supercond. Sci. Technol. 22 (2009) 125006; X. Xu, J. H.Kim, S. X. Dou, S. Choi, J. H. Lee, H. W. Park, M. Rindfleish, M.Tomsic, J. Appl. Phys. 105 (2009) 103913, as well as for the synthesisof FeSe superconductors with high superconducting phase content andimproved superconducting properties [S. Zhang, J. Liu, J. Feng, Ch. Li,X. Ma, P. Zhang, J. Materiomics 1 (2015) 118. Recently, the influence ofHEBM on the superconducting properties of Y-123 superconductors has beenstudied [A. Hamrita, Y. Slimani, M. K. Ben Salem, E. Hannachi, L.Bessais, F. Ben Azzouz, M. Ben Salem Ceramics International, 40 (2014)1461-1470—incorporated herein by reference in its entirety].

In view of the forgoing, one object of the present disclosure is toprovide methods for producing Y₃Ba₅Cu₈O_(y) (Y-358) with a novelmorphology/microstructure and thus improved superconducting properties.

BRIEF SUMMARY OF THE DISCLOSURE

Accordingly, it is one object of the present invention to provide novelmethods of producing polycrystalline Y₃Ba₅Cu₈O_(y) (Y-358) with a uniquemorphology and improved superconducting properties.

These and other objects, which will become apparent during the followingdetailed description, have been achieved by the inventors' discoverythat ball milling under controlled conditions provides polycrystallineY₃Ba₅Cu₈O_(y) with a unique morphology of elongated crystals withspherical nanoparticles disposed on the elongated crystals, and thepolycrystalline Y₃Ba₅Cu₈O_(y) material formed has been found to possesssuperior superconducting properties.

Therefore, according to a first aspect, the present disclosure relatesto a method of producing polycrystalline Y₃Ba₅Cu₈O_(y), involving (i)pelletizing powders of yttrium (III) oxide, a barium (II) salt, andcopper(II) oxide to produce a pelletized mixture, (ii) calcining thepelletized mixture at 850 to 950° C. to produce a calcined mixture,(iii) ball milling the calcined mixture to produce a ball milled sample,and (iv) sintering the ball milled sample at 900 to 1000° C. in anoxygen environment, where the final polycrystalline Y₃Ba₅Cu₈O_(y) is inthe form of elongated crystals having an average length of 2 to 10 μmand an average width of 1 to 2 μm embedded with spherical nanoparticleswith an average diameter of 5 to 20 nm.

In some embodiments, the barium (II) salt is barium (II) carbonateBaCO₃.

In some embodiments, the pelletized mixture is calcined twice at 850° C.to 950° C. for 8 to 16 hours.

In some embodiments, the calcined mixture is ball milled using aplanetary ball mill.

In some embodiments, the calcined mixture is ball milled using stainlesssteel balls and vials.

In some embodiments, the calcined mixture is ball milled with a ball topowder weight ratio of 1:1 to 5:2.

In some embodiments, the calcined mixture is ball milled at a rotationalspeed of 300 to 600 rpm.

In some embodiments, the calcined mixture is ball milled for 3 to 5hours.

In some embodiments, the calcined mixture is ball millednon-continuously with a paused time of 5 to 10 minutes every 20 to 30minutes in favor of cooling the system down and reverse rotation.

In some embodiments, the ball milled mixture is heat treated in anoxygen atmosphere at 900 to 1000° C. for up to 72 hours.

In some embodiments, the spherical nanoparticles generated by ballmilling process are present as agglomerates having a flower-likemorphology with an average particle size of 30 to 60 nm.

In some embodiments, the spherical nanoparticles generated by ballmilling process are uniformly dispersed on the elongated crystals.

In some embodiments, the polycrystalline Y₃Ba₅Cu₈O_(y) has a normalizedtransport critical current density. J_(ctN), of 0.040 to 0.044 under anapplied transverse magnetic field (μ₀H) of 100 mT.

In some embodiments, the optimized ball milled polycrystallineY₃Ba₅Cu₈O_(y) has a magnetization critical current density J_(cm), of12×10³ to 15×10³ A·cm⁻² and 550 to 570 A·cm⁻² at 0 Tesla and 1 Tesla,respectively.

In some embodiments, the polycrystalline Y₃Ba₅Cu₈O_(y) has a lowercritical magnetic field (B_(c1)(0)) of 7 to 7.25 Tesla and an uppercritical magnetic field (B_(c2)(0)) of 580 to 585 Tesla.

In some embodiments, the polycrystalline Y₃Ba₅Cu₈O_(y) has an estimatedcritical current density at temperature T=0K (J_(c)(0)) of 320×10³ to330×10³ A·cm⁻².

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is the X-ray powder diffraction patterns of the S0-hand grindedand S1, S2, S3 ball milled samples.

FIG. 2A is a low magnification SEM micrograph of the overview of S0-handgrinded sample.

FIG. 2B is a low magnification SEM micrograph of the overview of S1 ballmilled sample.

FIG. 2C is a low magnification SEM micrograph of the overview of S2 ballmilled sample.

FIG. 2D is a low magnification SEM micrograph of the overview of S3 ballmilled sample.

FIG. 3A is a high magnification SEM micrograph of the overview ofS0-hand grinded sample.

FIG. 3B is a high magnification SEM micrograph of the overview of S1ball milled sample.

FIG. 3C is a high magnification SEM micrograph of the overview of S2ball milled sample.

FIG. 3D is a high magnification SEM micrograph of the overview of S3ball milled sample.

FIG. 4A is a high magnification SEM micrograph showing nano-particlesgenerated by ball milling process embedded within Y-358.

FIG. 4B is a high magnification SEM micrograph showing nano-particleswith a flower-like morphology embedded within Y-358 with a 45 nm averagescale.

FIG. 5A is a plot of the temperature dependence of the normalizedelectrical resistivity for S0-hand grinded and S1, S2, S3 ball milledsamples.

FIG. 5B is the curves in the transition region from FIG. 5A.

FIG. 6A is a plot of dp/dT versus temperature and Δσ⁻² versus T for S0hand grinded sample.

FIG. 6B is a plot of dp/dT versus temperature and Δσ⁻² versus T for S1ball milled sample.

FIG. 6C is a plot of dp/dT versus temperature and Δσ⁻² versus T for S2ball milled sample.

FIG. 6D is a plot of dp/dT versus temperature and Δσ² versus T for S3ball milled sample.

FIG. 7A is a plot of the dependence of the normalized transport criticalcurrent densities J_(ctN)(H) with magnetic field for various Y-358samples.

FIG. 7B is a plot of the normalized transport critical current densitiesJ_(ctN)(H) of various Y-358 samples at 100 mT.

FIG. 7C is a plot of the dependence of normalized transport criticalcurrent densities J_(ctN)(H) with magnetic field for unmilled (handgrinded) and ball milled YBa₂Cu₃O_(d) (Y-123) and Y₃Ba₅Cu₈O_(y) (Y-358)samples.

FIG. 8A is a plot showing the temperature dependence of transportcritical current density J_(ct) in an applied magnetic field of 0 mT forhand grinded and ball milled Y-123 and Y-358 samples.

FIG. 8B is a plot showing the temperature dependence of transportcritical current density J_(ct) in an applied magnetic field of 2 mT forhand grinded and ball milled Y-123 and Y-358 samples.

FIG. 8C is a plot showing the temperature dependence of transportcritical current density J_(ct) in an applied magnetic field of 100 mTfor hand grinded and ball milled Y-123 and Y-358 samples.

FIG. 9A is a plot of magnetization critical current density J_(ct),estimated from magnetization loops versus magnetic field (M-H) at 77 Kfor hand grinded and ball milled Y-123 and Y-358 samples.

FIG. 9B is a plot of magnetization critical current density J_(ct), at amagnetic field of 0 and 1 Tesla at 77 K for hand grinded and ball milledY-123 and Y-358 samples.

FIG. 10A is a plot of magnetic field dependencies of flux pinning forcedensity F_(p)(H) for S0-hand grinded and S1 S2, S3 ball milled samples.

FIG. 10B is a plot of magnetic field dependence of the flux pinningforce density at 77 K for unmilled (hand grinded) and ball milled Y-123and Y-358 samples.

FIG. 11A is a logarithmic plot of excess conductivity Δσ as a functionof the reduced temperature ε for S0 hand grinded sample.

FIG. 11B is a logarithmic plot of excess conductivity Δσ as a functionof the reduced temperature c for S1 ball milled sample.

FIG. 11C is a logarithmic plot of excess conductivity Δσ as a functionof the reduced temperatures for S2 ball milled sample.

FIG. 11D is a logarithmic plot of excess conductivity Δσ as a functionof the reduced temperature c for S3 ball milled sample.

FIG. 12A is a plot of evolutions of critical magnetic fields (B_(c1)(0)and B_(c2)(0)) for different sintered samples.

FIG. 12B is a plot of evolutions of critical current density at T=0K,J_(c)(0), for different sintered samples.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all of the embodiments of the disclosure are shown.

As used herein, the words “a” and “an” and the like carry the meaning of“one or more”. Additionally, within the description of this disclosure,where a numerical limit or range is stated, the endpoints are includedunless stated otherwise. Also, all values and subranges within anumerical limit or range are specifically included as if explicitlywritten out.

As used herein, the terms “optional” or “optionally” means that thesubsequently described event(s) can or cannot occur or the subsequentlydescribed component(s) may or may not be present (e.g. 0 wt. %).

The phrase “substantially free”, unless otherwise specified, describesan amount of a particular component (e.g., a metal oxide), that whenpresent, is present in an amount of less than about 1 wt. %, preferablyless than about 0.5 wt. %, more preferably less than about 0.1 wt. %,even more preferably less than about 0.05 wt. %, relative to a totalweight of the composition being discussed, and also includes situationswhere the composition is completely free of the particular component(i.e., 0% wt.).

The term “comprising” is considered an open-ended term synonymous withterms such as including, containing or having and is used herein todescribe aspects of the invention which may include additionalcomponents, functionality and/or structure. Terms such as “consistingessentially of” are used to identify aspects of the invention whichexclude particular components that are not explicitly recited in theclaim but would otherwise have a material effect on the basic and novelproperties of the polycrystalline Y₃Ba₅Cu₈O_(y) or the methods formaking said material. The term “consisting of” describes aspects of theinvention in which only those features explicitly recited in the claimsare included and thus other components not explicitly or inherentlyincluded in the claim are excluded.

A weight percent of a component, unless specifically stated to thecontrary, is based on the total weight of the formulation or compositionin which the component is included. For example if a particular elementor component in a composition is said to have 8 wt. %, it is understoodthat this percentage is in relation to a total compositional percentageof 100%.

The terms “milled” vs. “unmilled” are used herein to differentiateproducts produced using the inventive ball mill-based methods (“milled”)vs. those produced by non-ball mill-based methods (i.e., handmilling/hand grinding with a mortar and pestle).

Methods

Besides the different molecular formula and thus atomic percentages ofY, Ba and Cu, polycrystalline Y₃Ba₅Cu₈O_(y) (“Y-358”) differs from otherYBCO family members such as YBa₂Cu₃O_(d) (“Y-123”) in that it possessesfive CuO₂ planes and three CuO chains, and has distinct characteristicsand properties. In the nominal chemical formula of Y₃Ba₅Cu₈O_(y), y isthe oxygen content that v in most cases between 17 and 19, mostpreferably 18. Y-358 refers to a polycrystalline material composed ofoxides of yttrium, barium, and copper in a 3:5:8 molar ratio, where thepurity of oxides of yttrium, barium, and copper is greater than 99 wt.%, preferably greater than 99.5 wt. %, preferably greater than 99.9 wt.%, most preferably 99.99 wt. %. In preferred embodiments, no metals arepresent (which can be in the form of elemental metals, metal oxides, ormetal salts) other than yttrium, barium, and copper. For example, thepolycrystalline Y-358 is preferably substantially free of or completelyfree of elements, oxides and/or salts of bismuth, tungsten, promethium,silver, and the like. In some embodiments, only the polycrystallineY-358 superconducting material is present, and the Y-358 is notcomposited or coated with other non-metal materials such as graphene tomake the superconducting material. Furthermore, when referencing theY-358 product produced by the methods described herein, it is to beassumed that the produced polycrystalline material contains at least80%, preferably at least 85%, preferably at least 90%, preferably atleast 95% of the Y-358 phase as determined by X-ray diffraction, and isfree of or is substantially free of other polycrystalline phases, e.g.,YBa₂Cu₃O_(y) (Y-123), Y₂BaCuO_(y) (Y-211), Y₇Ba₁₁Cu₁₈O_(y) (Y-7-11-18),YBa₂Cu₄Oy (Y-124), Y₂Ba₄Cu₇Oy (Y-247), and the like, unless specificallystated otherwise.

Perhaps due to the fundamental chemical differences between Y-358 andother YBCO family members (e.g., Y-123), it disclosed herein that thetechniques and process parameters effective for producing Y-358 withadvantageous superconducting properties differ from techniques andprocess parameters suitable for foil ling Y-123 [see A. Hamrita, Y.Slimani, M. K. Ben Salem, E. Hannachi, L. Bessais, F. Ben Azzouz, M. BenSalem, Ceramics International, 40 (2014) 1461-1470—incorporated hereinby reference in its entirety]. The inventive method for producingpolycrystalline Y-358 are disclosed below.

Starting Material Mixture

Yttrium (III) oxide, a barium (II) salt, and copper(II) oxide arepreferably used as starting materials. The barium (II) salt may be abarium oxocarbon anion or carboxylate e.g., barium carbonate (BaCO₃),barium cyclohexanebutyrate, barium 2-ethylhexanoate, barium octoate; abarium alkoxide e.g., barium methoxide, barium ethoxide, bariumisopropoxide; a barium halide e.g., barium bromide, barium chloride,barium fluoride, barium iodide; barium chromate; barium hydroxide;barium phosphate; barium metaphosphate; and the like, including mixturesthereof. In preferred embodiments, the barium (II) salt is bariumcarbonate (BaCO₃).

It is preferred that powders of the yttrium (III) oxide (Y₂O₃), barium(II) carbonate (BaCO₃), and copper (II) oxide (CuO) are used, and thatthe powders are mixed in an atomic percentage to provide a nominalcomposition of Y:3/Ba:5/Cu:8. While it is possible to introduce othermaterials, compounds, additives, etc. at this stage, the mixed powderpreferably consists of or consists essentially of only the yttrium,barium, and copper starting materials (i.e., respective oxides and/orsalts thereof), and thus it is preferred that no other materials,compounds, additives, etc. are present which would materially affect theability of the mixture to be pelletized, mechanically alloyed, andsintered as discussed hereinafter to form the polycrystalline Y-358. Forexample, the mixed powder is preferably free of potassium carbonate.Furthermore, while not always the case, it is preferred that only asingle source of each of yttrium, barium, and copper is employed in themethods herein, i.e., the only source of yttrium used to make thepolycrystalline Y-358 is yttrium (III) oxide, the only source of copperused to make the polycrystalline Y-358 is copper (II) oxide, and thereis only one source of barium (i.e., one barium (II) salt e.g., BaCO₃)used to make the polycrystalline Y-358. Other sources of yttrium (e.g.,yttrium carbonate, yttrium chloride, yttrium nitrate, yttriumtrifluoroacetate, yttrium acetate, yttrium acetoacetonate, etc.), othersources of copper (copper carbonate, copper chloride, copper nitrate,copper trifluoroacetate, copper acetate, copper acetoacetonate, copperbromide, copper hydroxide, etc.), and other sources of barium (e.g.,barium oxide), are preferably not employed as starting materials in thedisclosed inventive methods.

The powders of yttrium (III) oxide (Y₂O₃), barium (II) salt carbonate(e.g. BaCO₃), and copper (II) oxide (CuO) are mixed in accordance to thechemical formula Y:3/Ba:5/Cu:8 using an agate mortar and pestle.

Pressing/Pelletizing

After mixing, the mixed powders may be advantageously shaped byprocesses such as uniaxial pressing, isostatic pressing, molding,compacting, extrusion, injection, or any other pelletizing techniqueknown to those of ordinary skill in the art, to produce a pelletizedmixture. Preferably, the mixed powders are uniaxiallypressed/pelletized, meaning the compaction of powder into a rigid moldby applying pressure in a single axial direction through a rigid die orpiston. The mixed powder may be pelletized using hot uniaxial pressing(i.e., uniaxial pressing under the application of heat), for exampleunder a temperature of 800 to 1,500° C. as described in U.S. Pat. No.8,168,092B2. In most preferred embodiments, the mixed powders areuniaxially cold pressed, preferably at a temperature of less than 40°C., preferably less than 35° C., preferably 20 to 30° C. The mixedpowders may be pelletized under a wide range of applied pressures, forexample 5 to 700 MPa, preferably 10 to 600 MPa, preferably 20 to 300MPa, preferably 30 to 100 MPa, preferably 40 to 60 MPa. The shape of theproduced pellets is not particularly limiting, and can be adjusted byappropriate selection of the mold, die, and/or piston employed duringthe pelletizing processes, so long as a uniform density is achieved inthe pelletized mixture. A uniform density is desirable as it may aideven distribution of heat during the subsequent calcination process. Ifa uniform density is not provided by uniaxial pressing, then thepelletized mixture may optionally be isostatically pressed until thedesired density uniformity is achieved.

Calcination

The method involves calcining the pelletized mixture. In someembodiments, the calcination step is performed in a furnace using, forexample, a pre-set temperature program or using other variabletemperature systems known by those of ordinary skill in the art. In someembodiments, the pelletized mixture is calcined at 700 to 1,000° C.,preferably 800 to 975° C., preferably 850 to 950° C., more preferably875 to 925° C., more preferably 900° C. to produce a calcined mixture.In some embodiments, the pelletized mixture is calcined for 8 to 16hours, preferably 10 to 14 hours, preferably about 12 hours. Thepelletized mixture may be calcined using an isothermal procedure or anon-isothermal procedure. When a non-isothermal procedure is utilized,the temperature may be ramped using various ramp rates, for example 0.1to 10° C./min, preferably 0.3 to 8° C./ruin, preferably 0.5 to 7°C./min, preferably 0.8 to 6° C./min, preferably 1 to 5° C./min,including ramp rates outside of these ranges.

To ensure adequate removal of the barium salt anion (e.g., carbonateanion in BaCO₃), the calcined mixture may be optionally subjected to anintermediate grinding stage, for example with a mortar and pestle or amechanical grinding machine, and then re-subjected to furthercalcination under similar or different conditions to those describedabove. For example, the calcination process may involve 1) a firstcalcination at 850 to 950° C. for 10 to 14 hours, 2) intermediategrinding, followed by 3) a second calcination at 900 to 1,150° C. for 20to 24 hours. Various other modifications, such as changes of gaseousatmosphere, may also be practiced so long as suitable conversion of thebarium salt to barium oxide is adequately achieved. Most preferably, thepelletized mixtures are subjected to two calcinations processes, for 12hours at 900° C. with ramp rate of 5° C./min, each with intermediategrinding for the aim of producing an oxide precursor without residue ofany carbonates. The obtained precursors were divided into two parts; thefirst part was grounded by hand using an agate pestle in the agatemortar. The second part was ball milled.

Ball Milling

After producing calcined mixtures without residue of any carbonates, thecalcined mixtures are then ball milled to produce ball milled samples. Ahigh energy ball mill is preferably used, for example, a standard ballmill, a planetary mill, a vibration mill, an attritor—stirring ballmill, a pin mill, or a rolling mill may be employed. In someembodiments, the calcined mixture is ball milled with a planetary ballmill.

Planetary ball mills are typically used for grinding sample materialdown to very small sizes. A planetary ball mill includes at least onegrinding vial which is arranged eccentrically on a so-called sun wheel.The direction of movement of the sun wheel is opposite to that of thegrinding vials. The grinding balls in the grinding vials are subjectedto superimposed rotational movements, the so-called Coriolis forces. Thedifference in speeds between the balls and vials produces an interactionbetween frictional and impact forces, which releases high dynamicenergies. Besides the typical mixing result of ball milling, theinterplay between these forces in planetary ball milling produces thehigh and very effective degree of size reduction, and this high degreeof fineness can be accomplished using short milling times. An exampleplanetary ball mill useful in the disclosed method is a Retsch PM 200Ftype planetary ball mill, Haan Germany.

The vials and balls used for the ball milling may be one or more ofagate (cryptocrystalline silica), corundum (Al₂O₃), zirconium oxide(ZrO₂), stainless steel (Fe, Cr, Ni), tempered steel (Fe, Cr), andtungsten carbide (WC). In preferred embodiments, the balls are made ofstainless steel (e.g., SS 316), for example, 6 mm SS316 ball bearingsmay be employed. In some embodiments, the calcined mixture is ballmilled with stainless steel vials and balls.

In some embodiments, ball milling is performed in air (or a generallyoxygen-containing atmosphere, e.g., which includes any atmosphere thatcontains at least 20%, preferably at least 40%, preferably at least 60%,preferably at least 80%, preferably at least 90%, preferably at least95%, preferably at least 99%, or about 100% oxygen by volume). To avoidor reduce contamination, the ball milling may be carried out under aninert atmosphere such as under nitrogen or argon, preferably argon. Insome embodiments, the weight of the calcined mixture (g) per volume(1,000 cm³) of vials used in the ball milling is 50-150 g/1,000 cm³,preferably 75-130 g/1,000 cm³, preferably 90-110 g/1,000 cm³.

As used in the present disclosure, “controlled ball billing” or ballmilling under “controlled” conditions refers to a ball milling processthat maintains a consistent, stable, and highly reproducible millingevent by avoiding overly energetic milling parameters that can causeover-milling and over-heating of the ball system leading to unwantedmorphology changes and diminishment of superconducting properties. Thephrase controlled ball milling is not to be confused with low energyball milling methods, as high energy ball milling (e.g., planetary ballmilling) techniques may be employed herein under particular parameters.Parameters used herein for “controlled” ball milling are now describedbelow.

The ball to powder ratio (BPR) represents the weight ratio of themilling balls to the calcined mixture charge. The ball to powder ratioused herein may range from 1:1 to 4:1, preferably 3:2 to 7:2, preferably2:1 to 3:1, most preferably 5:2.

The calcined mixture may be ball milled at a rotational speed of 200 to600 rpm, preferably 250 to 550 rpm, preferably 300 to 500 rpm,preferably 350 to 450 rpm, most preferably 400 rpm.

The milling time may also influence the product morphology andsuperconducting properties of the produced polycrystalline Y-358material. Suitable milling times that may be practiced herein range from2 to 8 hours, preferably 2.5 to 6 hours, preferably 3 to 5 hours,preferably 3.5 to 4.5 hours, most preferably about 4 hours.

For controlled ball milling, it is also preferred to maintain arelatively low and consistent temperature within the ball millthroughout the milling operation, for example, a milling temperature ofless than 150° C., preferably less than 100° C., preferably less than80° C., preferably less than 60° C., preferably less than 40° C., forexample 20 to 35° C. The milling temperature herein is an averagetemperature within the mill, which can be measured by directcalorimetric measurements of the milling balls or vials, and not ameasurement of maximum local temperatures generated transiently duringimpact between the powder and/or colliding milling tools.

While the calcined mixture may in certain circumstances be ball milledcontinuously over the entire milling time (e.g., 4 hours), suchcontinuous operation may inadvertently cause elevated temperatures ortemperature spikes (uncontrolled milling temperatures) that maynegatively impact the final product. Therefore, in preferredembodiments, the calcined mixture is ball milled non-continuously overthe selected milling time. Such non-continuous operation typicallyentails ball milling in increments of 10 to 60 minutes, preferably 15 to50 minutes, preferably 20 to 40 minutes, preferably 25 to 30 minutes,separated by periods of inactivity (“cooling off periods”) ranging from1 to 15 minutes, preferably 5 to 10 minutes, to control and maintain amore consistent maximum milling temperature over the entire ball millingoperation. For example, a non-continuous ball milling operation mayinclude cycles of ball milling for 25 minutes, cooling off for 5minutes, then restarting the cycle again until a total milling time of 4hours is reached (i.e., the milling time is the sum of all activemilling periods and all cooling off periods).

On the other hand, uncontrolled ball milling, i.e., overly energeticball milling, generally involves application of a BPR, a rotationalspeed, a milling time, and/or a milling temperature that fall outside ofthe above described controlled ball milling ranges. For example, amethod that employs a BPR of 5:1 or more, a rotational speed of 700 rpmor more, a milling time of 10 hours or more, a milling temperature of200° C. or more, or any combination of two or more of these parameterswould be considered herein as “uncontrolled” ball milling.

In preferred embodiments, the calcined mixture which is subjected toball milling consists of or consists essentially of yttrium, barium, andcopper oxides, and a process control agent is not employed in order toavoid contamination during said ball milling process. Process controlagents that are typically excluded herein, include, but are not limitedto, organic acids or salts thereof (e.g., stearic acid, oxalic acid,benzoic acid, sodium stearate), polymers (e.g., polyvinyl alcohol,cellulose polymers such as sodium carboxymethyl cellulose, polyethyleneglycol), alcohols (e.g., methanol, ethanol, isobutyl alcohol),aluminum-containing compounds (e.g., aluminum tri-sec-butylate, aluminumchloride), alkanes (e.g., hexane), potassium carbonate, as well as anyother process control agent known by those of ordinary skill in the art.

Surprisingly, it has been found that methods utilizing a controlled ballmilling step afford polycrystalline Y-358 materials with superiorsuperconducting properties, while the use of uncontrolled ball millingmay negatively impact the superconducting properties of thepolycrystalline Y-358 material compared to unmilled samples. Therefore,as will become clear, methods utilizing controlled ball milling>othermilling techniques (e.g., hand grinding/milling)>uncontrolled ballmilling, in terms of making polycrystalline Y-358 material withdesirable superconducting properties. While not bound by theory, thismay be due to the morphology produced using controlled ball milling.

Sintering

After milling, the produced hand grinded and ball milled samples maythen be sintered, which is the process of compacting and forming a solidmass of material by heat or pressure without melting it to the point ofliquefaction. In some embodiments, the ball milled sample is sintered at800 to 1100° C., preferably 850 to 1050° C., preferably 900 to 1000° C.,most preferably at 950° C.

In preferred embodiments, the ball milled sample is sintered in anoxygen environment, which includes any environment that contains atleast 20%, preferably at least 40%, preferably at least 60%, preferablyat least 80%, preferably at least 90%, preferably at least 95%,preferably at least 99%, or about 100% oxygen by volume.

In some embodiments, the ball milled sample is sintered for up to 72hours, preferably 6 to 72 hours, preferably 12 to 60 hours, mostpreferably for 48 hours. Sintering may also optionally be performedabove atmospheric pressure, for example at a pressure of 200 to 900 MPa,preferably 500 to 800 MPa, preferably 700 to 775 MPa, most preferablyabout 750 MPa. After sintering and optionally cooling to roomtemperature, the polycrystalline Y-358 material is formed.

Product Morphology

The polycrystalline Y₃Ba₅Cu₈O_(y) material produced from the methodsdisclosed herein is in the form of a matrix of elongated grains/crystalswhich are separated by grain boundaries. The elongated crystals may havea variety of shapes, including cylindrical and cuboid shapes. In someembodiments, the elongated crystals have an average length of 1 to 20μm, preferably 1 to 15 μm, most preferably 2 to 10 μm and an averagewidth of 0.5 to 3 preferably 1 to 2.5 μm, most preferably 1 to 2 μm.

These elongated crystals may have an ordered or aligned orientation,whereby at least 60%, at least 70%, at least 80%, at least 90% of allelongated grains/crystals are arranged or oriented in the same orsubstantially the same direction as determined by SEM microscopy. Inpreferred embodiments, the matrix of the elongated grains/crystals arerandomly oriented, that is, the elongated crystals are generally notaligned or oriented in the same direction along their longitudinal axis(see FIGS. 2B and 3B).

The polycrystalline Y₃Ba₅CH₈O_(y) material produced from the methodsdisclosed herein also include spherical nanoparticles of Yttriumdeficient Y₃Ba₅Cu₈O_(y) disposed on the elongated crystals. Thespherical nanoparticles typically have an average diameter of 5 to 20nm, preferably 6 to 15 nm, preferably 7 to 12 nm, preferably 9 to 11 nm,or about 10 nm. “Dispersity” is a measure of thehomogeneity/heterogeneity of sizes of particles in a mixture. Thecoefficient of variation (CV), also known as relative standard deviation(RSD) is a standardized measure of dispersion of a probabilitydistribution. It is expressed as a percentage and may be defined as theratio of the standard deviation (σ) to the mean (μ, or its absolutevalue ∥μ∥), and it may be used to show the extent of variability inrelation to the mean of a population. In a preferred embodiment, thespherical nanoparticles produced by the methods of the presentdisclosure have a narrow size dispersion, i.e., are monodisperse, with acoefficient of variation of less than 30%, preferably less than 25%,preferably less than 20%, preferably less than 15%, preferably less than12%, preferably less than 10%, preferably less than 8%, preferably lessthan 5%, preferably less than 3%, with the coefficient of variationbeing defined in this context as the ratio of the standard deviation tothe mean diameter of the spherical nanoparticles.

Sphericity is a measure of how closely the shape approaches that of amathematically perfect sphere, and is defined as the ratio of thesurface area of a perfect sphere of the same volume to the surface areaof the spherical nanoparticles (with unity being a perfect sphere).Preferably, the spherical nanoparticles have an average sphericity of atleast 0.7, preferably at least 0.8, preferably at least 0.9, preferablyat least 0.95. In some embodiments, the spherical nanoparticles areclassified based on roundness, and are categorized herein as beingsub-rounded, rounded, or well-rounded, preferably well-rounded, usingvisual inspection similar to characterization used in the Shepard andYoung comparison chart (FIG. 4B).

In some embodiments, the spherical nanoparticles (microstructure) may bepresent on the elongated crystals in the form of agglomerates, wherein aplurality of spherical nanoparticles agglomerate, and thus shareinterconnected outer boundaries, to form a distinct agglomeratedmacrostructure. In preferred embodiments, the agglomerates have aflower-like morphology, such flower-like morphology being characterizedby the presence of individual spherical nanoparticles arranged radiallyin a petal-like manner surrounding centrally located sphericalnanoparticles, the centrally located spherical nanoparticles beinganalogous to the ovary of a flower (FIG. 4B).

In some embodiments, the flower-like agglomerates have an averageparticle size of 30 to 60 nm, preferably 35 to 55 nm, preferably 40 to50 nm, preferably about 45 nm. The average particle size of agglomeratesis measured according to an average of the largest particle dimensionsof the agglomerates, that is, the largest possible agglomerate particledimension is measured and averaged. In some embodiments, theagglomerates having the flower-like morphology are uniformly dispersedon the elongated crystals, that is, the spherical nanoparticles areclustered into agglomerates, and those agglomerates are spreadevenly/uniformly over the matrix of elongated crystals (e.g., FIGS.3B-3D, 4A and 4B).

Such flower-like agglomerates are different from the coral-likeagglomerates (about 100 nm in size), previously observed in Y-123materials [A. Hamrita, Y. Slimani, M. K. Ben Salem, E. Hannachi, L.Bessais, F. Ben Azzouz, M. Ben Salem Ceramics International, 40 (2014)1461-1470—incorporated herein by reference in its entirety].

Contrary to ball milling-based methods, the use of other I'm ins ofmilling/grinding (e.g., hand grinding with a mortar and pestle) formsonly elongated crystals of Y₃Ba₅Cu₈O_(y), with no observable sphericalnanoparticles (FIG. 3A). Furthermore, it has been found that the amountof spherical nanoparticles formed increases with increasingintensity/energy of ball milling (as can be seen in FIGS. 3B-3D).

Product Properties

The methods of the present disclosure produce Y-358 materials withsuperior superconducting properties, which can be clearly seen bycomparing the products produced by the inventive methods to those madeusing milling techniques other than ball milling. This may be due to themorphology produced with the controlled ball milling proceduresdisclosed herein, that is, the amount of spherical nanoparticles presenton the elongated crystals. One measurement for determining the quantityof spherical nanoparticles formed is residual resistivity, ρ₀, whichmeasures resistivity arising from impurity and imperfection scattering.Therefore, in the present disclosure, the residual resistivity may becorrelated to the quantity of spherical nanoparticles, with higherresidual resistivity values indicating a higher content of sphericalnanoparticles. In preferred embodiments, the Y-358 materials providedherein have a residual resistivity of 0.3 to 0.55 mΩ·cm, preferably 0.33to 0.5 mΩ·cm, preferably 0.36 to 0.48 mΩ·cm, most preferably 0.4 to 0.44mΩ·cm. Such residual resistivity values are indicative of Y-358materials having an advantageous content of spherical nanoparticles,while ρ₀ values above or below these ranges may be associated withpolycrystalline Y-358 materials having inferior superconductingproperties due to too many or too little (including none) sphericalnanoparticles.

In some embodiments, the Y-358 materials provided herein have anadvantageous intrinsic superconducting parameters, for example a lowercritical magnetic field, B_(c1)(0), of 5 to 10 T, preferably 5.5 to 8 T,preferably 6.0 to 7.5 T, most preferably 7 to 7.25 T, and an uppercritical magnetic field, B_(c2)(0), of 500 to 650 T, preferably 550 to650 T, preferably 570 to 600 T, most preferably 580 to 585 T. On theother hand, methods instead involving hand grinding or uncontrolled ballmilling have a B_(cl)(0) of 4.41 T and 2.51 T, respectively, and aB_(c2)(0) of 274 T and 109.6 T, respectively.

Critical current density is one measure of a materials ability to act asa superconductor, which is the point at which the vector sum of currentdensities is high enough to quench the superconducting state and thematerial transitions back to a normal state. In some embodiments, thepolycrystalline Y₃Ba₅Cu₈O_(y) produced by the methods herein has anestimated critical current density at temperature 0 K, J_(c)(0), of 290to 400×10³ A·cm⁻², preferably 300 to 370×10³ A·cm⁻², preferably 310 to350×10³ A·cm⁻², most preferably 320 to 330×10³ A·cm⁻². Such a criticalcurrent density compares favorably to un milled-based methods (e.g.,J_(c)(0) of 148.8×10³ A·cm⁻²) and methods employing uncontrolled ballmilling techniques (e.g., J_(c)(0) of 60.17×10³ A·cm⁻²).

Flux pinning force density, Fp (T·A/m²) is measurement of sensitivity tomagnetic field and flux pinning properties, with higher valuesindicating enhanced flux pinning and less sensitivity to magneticfields. The products provided by the inventive methods have a superiorflux pinning force density, F_(p), compared to both hand ground samplesand samples which have subject to over-milling (i.e., uncontrolled ballmilling), under all applied magnetic fields up to 800 mT (FIG. 10A).Furthermore, ball milling Y-358 under controlled conditions greatlyincreases F_(p) (e.g., more than a 2×10⁶ T·A/m² increase at 1 T),compared to unmilled Y-358, while there is less observed difference inF_(p) values between other yttrium-containing polycrystalline materials(e.g., Y-123) which have been subject to controlled ball milling andhand grinding (an increase of only about 0.5×10⁶ T·A/m² at 1 T, FIG.10B). This is unexpected, and demonstrates that the inventive methoddoes not enhance the superconducting properties of allyttrium-containing polycrystalline materials equally or to a similarextent.

Normalized transport critical current density, J_(ctN), whereJ_(ctN)=J_(ct)(B)_(/)J_(ct)(0), is another measure of sensitivity tomagnetic field and flux pinning properties, with higher valuesindicating enhanced flux pinning and less sensitivity to magnetic field.Methods disclosed herein utilizing a controlled ball milling stepproduce polycrystalline Y₃Ba₅Cu₈O_(y) materials having a J_(ctN) of0.039 to 0.045, preferably 0.040 to 0.044, preferably 0.041 to 0.043,most preferably about 0.042 under an applied transverse magnetic fieldμ₀H of 100 mT at 77K. It can be appreciated that controlled ball millingenhances the J_(ctN) value of polycrystalline Y-358 materials comparedto unmilled products, while over-milling (i.e., uncontrolled ballmilling) weakens the normalized transport critical current densityproperty of the material (FIGS. 7A-7B). Furthermore, the inventivemethod results in a clear improvement in J_(ctN) when applied to Y-358materials compared to unmilled variants, whereas there is no appreciableobserved difference in J_(ctN) values between ball milled and handground samples of other yttrium-containing polycrystalline materials,e.g., Y-123 (FIG. 7C). This is unexpected, and once again demonstratesthat the inventive method is suitable for enhancing the superconductingproperties of Y-358, while other YBCO materials do not necessarily showa similar improvement.

In some embodiments, the polycrystalline Y-358 product has an offsetsuperconducting transition temperature, T_(co), of 91.0 to 92.4,preferably 91.4 to 92.2, preferably 91.8 to 92.0 K.

The superconducting Y₃Ba₅Cu₃O_(y) materials produced in the presentdisclosure can be manufactured into rods, films, wires, tapes, coatings,coils, bulk materials, and the like, for various applications, such asin magnetic resonance imaging, particle accelerators, magneticlevitation, electric motors, bulk magnetic materials, electrical orcomputer components, power cables, power transmissions, transfer cablegenerators, frictionless bearings, Josephson junctions, and the like.

Having generally described this disclosure, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified. The examples below are intended tofurther illustrate methods of preparing and characterizing thepolycrystalline Y₃Ba₅Cu₈O₅, materials and they are not intended to limitthe scope of the claims.

EXAMPLES Experimental

Products of Y-358 were elaborated by solid state reaction by using twodifferent milling methods, hand grinding in a mortar and ball milling ina planetary crusher. In accordance to the chemical formula ofY:3/Ba:5/Cu:8, the starting powders of Y₂O₃, BaCO₃, and CuO were mixedin an agate mortar by hand grinding using an agate pestle. The mixturewas pressed uniaxially into pellets under an applied pressure of 100 MPaand thereafter subjected to two calcinations processes for 12 h at 900°C. each with intermediate grinding for the sake of producing an oxideprecursor without residue of any carbonates. The obtained precursor wasdivided into two parts; the first part was grounded by hand per an agatepestle in the agate mortar. The second part was milled using Retsch PM200F type planetary ball milling technique with stainless steel ballsand vial for 4 hours with various processing parameters. The number ofballs, the ball to powder weight ratio and the speed rotation werevaried. The process of milling was paused every 25 min for 5 min infavor of cooling the system down and reverse rotation. The precursorspowders were then pelletized under an applied pressure of 750 MPa andheated in an oxygen atmosphere for 48 h at 950 The samples elaboratedthrough sintering of planetary ball-milled precursor powder are named asS1, S2 and S3. For hand grinded precursor, the sintered sample is namedS0. The notation and experimental variables for all elaborated productsare summarized in Table 1.

TABLE 1 Notation and experimental conditions of ball milling parameters.Processing ball-milling Sample notation parameters S0 S1 S2 S3 Number ofballs — 3 3 4 Ball-to-powder weight ratio — 5:2 5:2 5:1 Rotation speed —400 600 600

Product Properties XRD and SEM Characterizations

XRD data (FIG. 1 ) confirms the formation of a mainly Y-358 single phasewith a small quantity of secondary phases identified with (*) and (+) inspectrum.

FIGS. 2A-2D showed that the average grain size is smaller for ballmilled samples compared to hand grinded one meaning that the planetaryhigh energy ball milling technique has a significant impact on themicrostructure of sintered samples. The different samples are formed byelongated crystals that have an average length of 2 to 10 μm and anaverage width of 1 to 2 μm.

The images of FIGS. 3B-3D, taken under higher magnification, showedbright nanometer scale entities with almost regular form dispersed onthe matrix relating to the milled samples. Such nano-entities are notvisible in hand grinded sample (FIG. 3A). The density of thesenano-entities is raised on increasing the weigh ball-powder proportionand the speed rotation.

A closer look at much higher magnification (FIG. 4A) shows a sphericalshape nano-entities with a size of about 10 nm dispersed finely andrelatively uniformly within the matrix. These nano-entities sticktogether forming flower-like agglomerates (FIG. 4B).

The clumping of fine grains incorporated in the superconductor will leadto more disorder in the crystal lattice that expect to amplify thevortices pinning centers and consequently improve the transportproperties of the synthesized products.

Characterization of Superconducting Properties

The methods of the present disclosure produce Y-358 materials withsuperconducting properties. Varying the number of balls, the rotationspeed and the ball-to-powder weight ratio cause considerable disturbanceof the microstructure in samples induced either by interfaces,heterogeneities or created defects. These differences manifest intodifferences in various superconducting parameters (FIG. 5A and Table 2).

TABLE 2 Characteristic parameters of different sintered samples.Characteristic parameters Sample T_(co) T* T_(c) ΔT ρ_(n) ρ₀ notation(K) (K) (K) (K) (mΩ · cm) (mΩ · cm) S0 92.68 118.22 93.24 0.84 0.98 0.29S1 91.85 140.31 92.99 1.49 0.71 0.41 S2 91.71 148.52 92.88 1.51 0.790.53 S3 91.17 161.88 92.87 2.10 1.28 0.98

The increase in the residual resistivity, ρ₀, can be explained by thelarger number of defects and heterogeneities induced by planetary highenergy ball milling technique. Therefore, in the present disclosure, theresidual resistivity may be correlated to the quantity of sphericalnanoparticles, with higher residual resistivity values indicating ahigher content of spherical nanoparticles. The milled samples exhibithigher residual resistivity compared to the hand grinded one. Theresidual resistivity increases with increasing the various processingparameters such as the number of balls, speed rotation and the number ofball-powder.

The hand grinded sample (S0) exhibits a single transition to thesuperconducting state, however, the milled ones exhibit a doublesuperconducting transition (FIG. 5B). This can be confirmed by plottingdp/dT versus temperature or Δσ⁻² versus temperature (FIG. 6A-6D). Forexample, the presence of one maximum in plots of dp/dT versustemperature for the S0 unmilled sample indicates the occurrence of asingle transition to the superconducting state (FIG. 6A), however theobservation of two maximums for milled samples indicates the presence ofa double superconducting transition (FIG. 6B-6D). It is assumed that thedouble transition, which is not observed in the case of hand grindedsample, seems to arise from the nano-entities generated by the planetaryhigh energy ball milling technique.

Normalized transport critical current density, J_(ctN), whereJ_(ctN)=J_(ct)(B) J_(ct)(0), is a measure of sensitivity to magneticfield and flux pinning properties, with higher values indicatingenhanced flux pinning and less sensitivity to magnetic field. It can beappreciated that controlled ball milling enhances the J_(ct); value ofpolycrystalline Y-358 materials compared to hand grinded products. Theoptimized ball milled sample (S1) exhibits less sensitivity to magneticfield and J_(ctN) values are higher throughout the considered wholerange of magnetic field when compared to those of hand grinded (S0) andother ball milled (S2 and S3) samples (FIG. 7A). The optimized ballmilled polycrystalline Y₃Ba₅Cu₈O_(y) material has a J_(ctN) of 0.042under an applied transverse magnetic field μ₀H of 100 mT at 77K, whichis higher compared to that for hand grinded samples where J_(ctN) isaround 0.026 (FIG. 7B). The nano-entities induced by the use ofappropriate and well-controlled ball milling parameters could act asefficient pinning sources resulting in a global improvement of fluxpinning properties.

In comparison to hand grinded and ball milled Y-123, the hand grindedand ball milled Y-358 samples exhibit higher values of critical currentdensity proving better intrinsic superconducting properties in Y-358compounds (FIG. 7C). The Y-358 samples exhibit less sensitivity tomagnetic field and J_(ctN) values are higher throughout the consideredwhole range of magnetic field when compared to those of otheryttrium-containing polycrystalline materials, e.g., Y-123 (FIG. 7C). Theobtained result reveals that the optimized ball milled Y-358 productdisplays the better flux pinning characteristics. This is unexpected,and once again demonstrates that the inventive method is suitable forenhancing the superconducting properties of Y-358, while other YBCOmaterials do not necessarily show a similar improvement.

All samples exhibit an improvement of J_(ct)(T) in the whole temperaturerange between close to T_(co) and down to T=20K. J_(ct)(T) values ofmilled samples are drastically better in the existence of externalmagnetic fields compared to unmilled ones. This result confirms onceagain the beneficial effect of well-dispersed nano-entities induced byplanetary HEBM technique in the enrichment of the flux pinningproperties. The performances of milled Y-358 product are better over theentire temperature range (FIGS. 8A-8C).

The hand grinded Y-358 product displays noticeably higher magnetizationcritical current density, J_(cm)(H), values in comparison to the handgrinded Y-123, at least in part due to better intra-granularcharacteristics in Y-358 (FIG. 9A). J_(cm) is improved significantly inthe ball milled Y-358 sample compared to ball milled Y-123 at both 0 and1 Tesla (FIG. 9B). The high energy ball milling technique leads to alarger improvement of magnetization critical current density for theY-358 compound compared to the Y-123 ball milled variant. In someembodiments, the optimized ball milled polycrystalline Y₃Ba₅Cu₈O_(y) hasa magnetization critical current density J_(cm) of 14×10³ A·cm⁻² and 550A·cm⁻² at 0 Tesla and 1 Tesla, respectively. However, the optimized ballmilled polycrystalline YBa₂Cu₃O_(d) has lower J_(cm) with values around7.0×10³ A·cm⁻² and 200 A·cm⁻² at 0 Tesla and 1 Tesla, respectively.

The S1 milled sample exhibits less sensitivity to magnetic field andflux pinning force density F_(p) values are higher throughout theconsidered whole range of magnetic field when compared to those of S0,S2 and S3 materials (FIG. 10A). That is to say, the flux pinning in S1milled sample has been enhanced compared to the other products. Thenano-entities induced by the use of appropriate and well-controlled ballmilling parameters could act as efficient pinning sources resulting in aglobal improvement of flux pinning properties.

The milled Y-358 exhibits a distinctly higher flux pinning force densityF_(p) and the planetary high energy ball milling technique once againstrengthens the flux pinning properties of this product in comparison tothat of Y-123 which shows a much smaller observed difference betweenmilled and unmilled Y-123 materials (FIG. 10B).

TABLE 3 Conductivity exponents and cross-over temperatures values fordifferent sintered samples. Sample T_(SWF−1D) T_(1D−2D) T_(2D−3D) T_(G)notation λ_(SWF) λ_(1D) λ_(2D) λ_(3D) λ_(cr) (K) (K) (K) (K) S0 2.971.51 1.04 0.47 0.34 122.92 109.8 102.92 94.79 S1 2.94 1.46 0.96 0.480.30 149.18 129.04 111.9 97.88 S2 2.95 1.53 0.96 0.47 0.21 160.16 137.52115.8 98.38 S3 2.95 1.54 0.96 0.47 0.22 173.31 145.84 119.84 99.06

TABLE 4 Physical parameters associated with fluctuation-inducedconductivity (FIC). J_(c)(0) Sample N_(G) × ζ_(c)(0) d s B_(c)(0)B_(c1)(0) B_(c2)(0) (×10³ notation 10⁻² (Å) (Å) (Å²) J (Tesla) (Tesla)(Tesla) A · cm⁻²) S0 1.63 10.96 87.12 1008.87 0.0632 24.10 4.41 274.17148.80 S1 6.31 7.50 32.53 195.62 0.2126 43.32 7.24 584.39 329.57 S2 4.849.44 35.98 254.49 0.2753 32.75 6.04 368.47 237.17 S3 6.67 17.32 65.68815.73 0.2781 12.18 2.51 109.64 60.17

The excess conductivity investigation of Y₃Ba₅Cu₈O_(y) superconductorsprepared by various parameters of planetary High Energy Ball Millingtechnique were examined (FIGS. 11A-11D, FIGS. 12A-12B, and Tables 3 and4). In FIGS. 11A-11D, the abbreviations C.R., M.F.R. and S.W.F. standfor critical region, mean-field region and short-wave fluctuationregion, respectively.

The lower critical magnetic field B_(c1)(0)), upper critical magneticfield (B_(c2) (0)) and critical current density at T=0K (J_(c)(0)) areimproved for S1 and S2 ball milled samples compared to S0 unmilled (handgrinded) sample. It is noticeable that the enhancement of theseparameters reaches the highest values for S1. The structure ofmulti-layers and the existence of greater number of insulating layersinterpolated between the planes CuO₂ playing as efficient intrinsicpinning centers and the generated nano-entities by high energy ballmilling technique may conduce together to much better intrinsic pinningcapabilities in Y-358. The nano-entities induced by the use ofappropriate and well-controlled ball milling parameters in Y-358 samplecould act as additional and efficient pinning sources resulting in aglobal improvement of flux pinning properties.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present disclosure. As will be understood by thoseskilled in the art, the present disclosure may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the disclosure, as well as other claims. The disclosure, includingany readily discernible variants of the teachings herein, defines, inpart, the scope of the foregoing claim terminology such that noinventive subject matter is dedicated to the public.

1. A method for making a YBCO superconductor having two superconductingtransitions, comprising: mixing powders of yttrium (III) oxide, bariumcarbonate, and copper(II) oxide then pelletizing to form a pelletizedmixture; calcining the pelletized mixture at a first temperature in therange 850-900° C. to form a first calcined mixture; grinding the firstcalcined mixture to form a first intermediate calcined mixture, thencalcining the first intermediate calcined mixture at a temperature inthe range 850-900° C. to form a second calcined mixture, wherein thesecond calcined mixture is free from carbonate; ball milling the secondcalcined mixture at a ball to powder ratio of 5:2 to produce a ballmilled sample; and sintering the ball milled sample at a secondtemperature in the range 900-1000° C. to form a polycrystallineY₃Ba₅Cu₈O_(y) material, wherein the polycrystalline Y₃Ba₅Cu₈O_(y)material is in the form of elongated crystals having an average lengthof 2 to 10 μm and an average width of 1 to 2 μm, and are embedded withspherical nanoparticles of yttrium deficient Y₃Ba₅Cu₈O_(y) having anaverage diameter of 5 to 20 nm, wherein the spherical nanoparticles arein the form of flower-like agglomerates having an average particle sizeof 30 to 60 nm.
 2. (canceled)
 3. The method of claim 1, wherein thepelletizing includes uniaxially pressing a mixture of the yttrium (III)oxide, the barium carbonate salt, and the copper(II) oxide under anapplied pressure of about 100 MPa.
 4. (canceled)
 5. The method of claim1, wherein the second calcined mixture is ball milled using a planetaryball milling technique.
 6. The method of claim 1, wherein the secondcalcined mixture is ball milled with stainless-steel balls and vials. 7.The method of claim 1, wherein the second calcined mixture is ballmilled with a ball to powder weight ratio of 1:1 to 5:2.
 8. The methodof claim 1, wherein the second calcined mixture is ball milled at arotational speed of 300 to 600 rpm.
 9. The method of claim 1, whereinthe second calcined mixture is ball milled for 3 to 5 hours.
 10. Themethod of claim 1, wherein the second calcined mixture is ball millednoncontinuously in increments of 20 to 30 minutes separated by coolingoff periods of 5 to 10 min.
 11. The method of claim 1, wherein the ballmilled sample is pelletized under a uniaxial pressure of about 750 MPa.12. The method of claim 1, wherein the ball milled sample is sintered inan oxygen atmosphere at 950° C. for up to 72 hours. 13-15. (canceled)16. The method of claim 1, wherein the polycrystalline Y₃Ba₅Cu₈O_(y)material has a normalized transport critical current density, JctN, of0.040 to 0.042 under an applied transverse magnetic field (μ₀H) of 100mT.
 17. The method of claim 1, wherein the polycrystalline Y₃Ba₅Cu₈O_(y)material has a magnetization critical current density J_(cm) of 13×10³to 15×10³ A·cm⁻² and 550 to 570 A·cm⁻² at 0 Tesla and 1 Tesla,respectively.
 18. The method of claim 1, wherein the polycrystallineY₃Ba₅Cu₈O_(y) material has a lower critical magnetic field (B_(c1)(0))of 7 to 7.25 Tesla and an upper critical magnetic field (B_(c2)(0)) of580 to 585 Tesla.
 19. The method of claim 1, wherein the polycrystallineY₃Ba₅Cu₈O_(y) material has an estimated critical current density attemperature T=0K J_(c)(0)) of 320×10³ to 330×10³ A·cm⁻².
 20. (canceled)